An internal combustion engine for an automobile is provided with various kinds of auxiliary devices such as an alternator, a compressor for an air conditioner, a cooling water pump. These devices are indirectly driven through a belt or directly driven by the crank shaft of the engine. Thus, each of the auxiliary devices is provided with a driven pulley so as to rotate the shaft of the device.
FIG. 4 shows a compressor 21 for compressing cooling medium for an air conditioner as an example of the auxiliary devices. The figure shows an example in which a swash plate 23 fixed to a rotation shaft 22 is rotated thereby to reciprocally move opposite-type pistons 25 sandwiched at their both side surfaces by balls 24 within cylinders 27 formed in a housing 26. The rotation shaft 22 is supported by needle bearings 28, 29 at the center portion of the housing 26.
A plate spring 31 is attached to the end portion of the rotation shaft 22 through an attachment bracket 30. An annular plate 32 made of magnetic material is fixed at the tip end of the plate spring 31. In the example shown in this figure, a driven pulley 37 with a U-sectional shape is supported through a bearing 36 at the outer periphery of a supporting cylindrical portion 35 protruded from the front head 33 of the compressor 21. A solenoid 38 fixed on the front head 33 side is disposed in a space of the U-sectional shape and the annular plate 32 made of magnetic material is disposed at a position opposing through the annular wall portion 39 of the driven pulley 37 to the solenoid 38, thereby constituting an electromagnetic clutch 40.
In the electromagnetic clutch 40, when the solenoid 38 is not supplied with a current, the annular plate 32 is separated from the annular wall portion 39 of the driven pulley 37 as shown in the figure. Thus, even when the driven pulley 37 is rotated by an endless belt, the annular plate 32 does not rotate and hence the compressor 21 does not operate. In contrast, when the solenoid 38 is supplied with a current, the annular plate 32 made of magnetic material is attracted by the magnetic force of the solenoid and so urged against the annular wall portion 39, whereby the electromagnetic clutch 40 is placed in a coupling state. Thus, the driven pulley 37 rotates, then the annular plate 32 also rotates integrally with the driven pulley thereby to rotate the swash plate 23 through the plate spring 31, the attachment bracket 30 and the rotation shaft 22, whereby the piston 25 is reciprocally moved thereby to operate the compressor 21.
In such a compressor for a vehicle, various kinds of the bearings 36 for supporting the driven pulleys 37 are employed. Conventionally, most of the compressors for vehicles employ double-row radial ball bearings 41 as shown in FIG. 3(a). FIG. 4 shows an example in which such a double-row radial ball bearing is used. In this bearing, double rows of balls 48, 49 are disposed between the double rows of outer raceways 43, 44 formed on the inner peripheral surface of an outer ring 42 and the double rows of inner raceways 46, 47 formed on the outer peripheral surface of an inner ring 45 in opposite to the outer raceways 43, 44, respectively. The balls of the respective rows are held in retainers 50, 51 with a predetermined interval, respectively. Seals 52, 53 are provided at the side portions of the balls 48, 49, respectively, so as to seal in a manner that grease within the seals does not leak therefrom and water, dust etc. does not enter within the seals from the outside.
As described above, in the case of using the conventional double-row radial ball bearing as a bearing for the pulley of a compressor, even when a small amount of an eccentric load is applied to the driven pulley 37 from the endless belt wound around the driven pulley 37, there scarcely arise such a case where the center axis of the outer ring 42 and the center axis of the inner ring 45 constituting the bearing 40 do not coincide to each other and so the bearing inclines. In particular, when the bearing is configured as an angular bearing as shown in FIG. 3(a), the bearing can even cope with a large eccentric load. Thus, sufficient durability of the bearing can be secured, and further the rotation center of the driven pulley 4 is prevented from inclining thereby to also prevent eccentric wear of the endless belt.
However, in the case of using the conventional double-row radial ball bearing, the bearing including the balls each having a relatively large diameter is used in order to surely receive a large load. As a result, the bearing inevitably becomes large in its size and so the width of the bearing along the axis line direction thereof also becomes large inevitably. However, the devices for a vehicle are demanded to be as small as possible in their weights and sizes. Thus, when the width of the bearing becomes large, various members such as supporting members for the bearing become large and so heavy and further become bulky as a whole, undesirably.
As a countermeasure of such a phenomenon, it is considered to use a bearing including balls each having a small diameter in order to make the conventional double-row ball bearing smaller thereby to reduce the width of the bearing along the axis line direction thereof. In this case, since the durability is degraded as the diameter of the ball becomes smaller, it has been thought that the miniaturization of the double-row ball bearing is limited. Thus, it has been thought that it is suitable to use a single-row deep groove ball bearing in order to narrow the width of the bearing, and a research is made as to the improvement of the single-row deep groove ball bearing.
That is, in the case of using a normal single-row radial ball bearing as the bearing for a driven pulley configured in the aforesaid manner, when the driven pulley 37 receives an eccentric load, the bearing has not a sufficient force for preventing the inclination of the driven pulley 37, whereby the degree of deviation or inconsistency between the center axis of the outer ring and the center axis of the inner ring constituting the radial ball bearing becomes remarkable. As result, not only the durability of the radial ball bearing becomes insufficient but also remarkable eccentric wear occurs at the endless belt wound around the driven pulley 37. In view of this point, a research is made to configure the single-row deep groove ball bearing as a four-point contact type.
According to such a research, as shown by an enlarged diagram of FIG. 3(b), an outer raceway 56 formed on the inner peripheral surface of an outer ring 55 is configured by two loci, that is, a first outer raceway 58 which is disposed on the right side of a ball 57 in the figure and has a curvature radius R1 larger than the curvature radius r of the ball 57 and a second outer raceway 59 which is disposed on the left side and has the same curvature radius, whereby the outer raceway 56 is configured in a so-called Gothic arch shape. Similarly, an inner raceway 61 formed on the outer peripheral surface of an inner ring 60 is configured by two loci, that is, a first inner raceway 62 and a second inner locus 63 each having a curvature radius R2 larger than the curvature radius r of the ball 57, whereby the inner locus also forms a locus of a so-called Gothic arch shape. Accordingly, the ball 57 contacts with the four points of the respective loci. In this bearing, seals 64 and 65 are also provided on the both sides of the ball, respectively.
Such a radial ball bearing of a four-point contact type has a larger rigidity with respect to an eccentric load as compared with a general single-row deep groove radial ball bearing and so the center axis of the outer ring 55 hardly deviates from the center axis of the inner ring 60 even when an eccentric load is applied thereto. An example where such a radial ball bearing of a four-point contact type is applied to a driven pulley for a compressor with an electromagnetic clutch is disclosed in JP-A-11-210766, for example, and also disclosed in other many known documents. As described in the above-publication, the improvement can be made by a three-point contact type as well as the aforesaid four-point contact type as compared with the conventional bearing.
As described above, although the problem caused by an eccentric load applied to the pulley is solved by using the double-row ball bearing, it has been thought that the usage of the double-row ball bearing is limited since a bearing with a narrower width in the axis line direction is required. Thus, a research is made as to the usage of the single-row ball bearing and so a research is made as to the aforesaid ball bearing of a four-point contact type etc. However, even when the single-row ball bearing is used, the diameter of a ball is required to be smaller in order to obtain a bearing with a narrower width.
However, it was found that, in the single-row ball bearing, a volume of vibration sound of the belt became larger when the diameter of the ball was made smaller. Many experimentations were repeatedly made as to the relation between the vibration sound and the diameter of the ball, whereby it was found that a volume of the vibration sound became large when the diameter of the ball becomes 5 mm or less, particularly. This result of the experimentations is shown in a table 1. This table shows the diameters of the ball where a volume of the vibration sound particularly changed when the diameter of the ball of the single-row ball bearing was changed. As clear from this table, in the single-row ball bearing, a volume of the vibration sound increases when the diameter of the ball reduces to about 5 mm, and then a volume of the vibration sound is large and so the bearing is not suitable to be mounted on a vehicle when the diameter of the ball is 4 mm. Thus, a ball with a small diameter such as 4 mm, 3 mm can not be used and so it was found that the reduction of the width of the bearing in the axis line direction is limited in this view point.
TABLE 1diameter of ball ofvolume of vibrationsingle-row ball bearingsound of belt6small5middle4large